<style>
  body {
    /* 没有这个属性看不出第一个小三角 */
    background-color: #999;
  }

  .box {
    margin: 40px;
    padding: 50px;
    background-color: #fff;
    position: relative;
    font-size: 24px;
  }

  .cor {
    position: absolute;
    left: -40px;
    width: 0;
    height: 0;
    overflow: hidden;
    border: 20px solid transparent;
    border-right-color: #fff;
  }

  .box-shadow {
    box-shadow: 5px 5px 10px black;
  }

  .drop-shadow {
    filter: drop-shadow(5px 5px 10px black);
  }
</style>

<div class="box box-shadow">
  <i class="cor"></i>
  box-shadow
</div>
<div class="box drop-shadow">
  <i class="cor"></i>
  filter: drop-shadow
</div>